This invention relates generally to moving armature magnetic transducers, and more particularly to armature assemblies designed to facilitate the positioning of a vibratory portion of the armature relative to the pole faces which establish a permanent magnetic flux.
The armature in these transducers is generally formed from strip material and extends between a pair of spaced magnetic poles having flat, parallel facing surfaces each forming a gap therewith. It is generally important for an armature portion to be parallel to the facing pole surfaces. It is also important to locate the surfaces of the armature portion relative to the respective facing pole surfaces. Both of these conditions affect the intensity and distribution of varying magnetic flux during operation of the transducer. The difficulties of establishing these locations are compounded for applications requiring very small physical dimensions as in hearing aids.
Earlier designs relied on manufacturing the transducer to close dimensional tolerances, so that when assembled the parts approximate the desired relative locations. Alternatively, various methods have been devised for adjusting the relative positions of the parts after assembly, as in U.S. Pat. Nos. 4,410,769 and 4,518,831. In these methods the elements of moving armature transducers have been chosen according to suitability for particular methods of post-assembly adjustment. In turn, in these prior art post-assembly adjustable structures, the configuration of the overall motor unit of the transducer tends to confer a rather high degree of rectangularity on the cross-sectional shape of the complete transducer.
When a blockier cross-sectional shape of the transducer is desired for particular end applications, a folded armature motor unit typically is better adapted to this need.
U.S. Pat. No. 3,185,779 to one of the present applicants originated the folded armature transducer in the prior art. A folded elongate armature having the end of one arm fixed to a plate as by spot welding has the vibratory end of the other arm extending into a space between two spaced magnets. The magnets are attached in fixed relation to the plate during assembly of the transducer. The accuracy of position of the vibratory end of the armature in the magnet space depends largely upon the precision to which it is practicable to form and heat treat the folded armature, starting its fabrication typically with a flat strip of material. Similar considerations apply to subsequent prior art designs employing folded armatures.
In these now conventional prior art folded armature transducers, it is not generally practicable to adjust the armature position mechanically after assembly. Also, during assembly it is generally not practicable to vary the location of the armature in a manner which minimizes the amount of required post-assembly adjustment by whatever method, such as differential demagnetization of the magnet pair.
For the purpose of improving the precision and ease of controlling the position of the vibratory end of a folded armature in relation to the pole faces, and at the same time reducing manufacturing costs, the features of this invention include a folded armature having integral wings extending laterally from the supported portion of the armature, the wings being formed to provide mutually parallel pads extending normal to the supported portion. The supported portion and the laterally extending portions of the wings together form a bridge between the pads. The parts forming the permanent magnetic flux means comprise a pair of spaced parallel, initially unmagnetized magnets affixed to the inner surfaces of a closed loop magnet strap which has a parallel set of outer surfaces to fit between the pads. In assembly, the magnet strap is fitted slidably between the pads with a clearance from the bridge of the armature and with the vibratory end of the armature extending between the magnets.
Prior to this assembly, however, an electrical coil having a hollow bore is attached as by adhesive to the magnet strap, its bore being aligned with the magnet strap""s aperture. During assembly the vibratory end of the armature is threaded through the bore of the coil to extend between the magnets.
In the assembly process, the position of the pads on the magnet strap is slidably adjusted to vary the position of the vibratory end of the armature, both rotationally to bring it into parallelism with the facing surfaces of the magnets, and translationally with respect to its spacing between them. After the desired location is established, the pads of the armature are permanently attached to the magnet strap as by laser welding. At that point a drive pin may be attached to the vibratory end of the armature slightly beyond the magnet strap, and then the magnets are magnetized, as by a unidirectional magnetic field pulse of external origin, thereby establishing a permanent magnetic flux in the magnet space and defining a magnetic center between the pole faces of the magnets where the vibratory end of the armature should be ideally located.
A feature of this invention is that the novel configuration of the armature, and the spacing between the bridge of the armature and the magnet strap, permit adjustment of the armature to or toward magnetic center by plastic deformation of the bridge of the armature extending between its pads. In response to this adjustment the vibratory end of the armature is moved essentially in translation relative to the pole faces of the magnets so as to approximately maintain its intrinsic parallelism thereto.